Green CovR preliminary data December 2020

Data Collection

In the fall of 2020, about 100 photos were submited from 7 counties. Becker, East Otter Tail, and Faribault each submitted more than 20 photos, Mower submitted 13, and there were a few from Olmsted, Traverse, and Winona. I want to examine the relationship between cover crop planting date and % ground cover.

Average green ground cover was 28%, but the range was all the way from 0-99, witha quarter of the phtoos showing less than 5% ground cover, and a quarter showing more than 50% ground cover. As the photos below demonstrate, some of the low numbers may be due to interference with residue, so they may represent very effective soil protection even at low values. The photo showing 65% green cover is also shows that you don't need to get anywhere near 100% to have excellent growth. Points to keep in mind as we walk through the data exploration.

Oat-radish-turnip mixture, 8% coverage and Oat-field pea-radish mixture, 64% coverage

Here's the basic relationship: % ground cover by planting date. The relationship between planting date and coverage is pretty clear, but there's a number of complicating factors: location, whether CCs were interseeded,and what crops they followed.

Effect of interseeding

The figures below show the effects of intereseeding and location. Above, you can see that there's a the strong relationship between planting date and coverage only when cover crops were not interseeded. When cover crops were interseeded, there's not much increase in ground cover with earlier seeding dates.

Looking at just the means of interseeding vs not, that there's no consistent advantage or disadvantage to interseeding. This probably reflects the various conditions and methods of seeding for both interseeded and non. Also, note that in most cases there's much fewer interseeded fields, so it's not a perfect comparison of the two methods.

Both figures show that in every county, there's a fair amount of variation, with no strong regional trends. For the four counties with more than 10 data points, Becker averaged 33% ground cover, East Ottertail 26%, Faribault 23%, and Mower 31%.

Effect of cover crop species

The question of cover crop species is a bit more complicated, as many of these winter-killed cover crops were mixes of multiple species. We collected up to 4 species included in mixes, but did not include estimates of what the proportion of each species was seeded or emerged. So there's a limit to how much we can say about the effect of species here.

Focusing on the grass species, as they are usually dominant, below I've graphed the mean % green cover of mixes which included oats, cereal rye, or annual rye. A few fields in EOT included both oats and cereal rye, but most fields included only one of these grasses.

Cover crop species also doesn't explain all the variability, although oats have maybe a slight advantage in Faribault county. The "other" data points include several spring wheat fields in Becker county, which appears to be a good option there, and a few winter triticale.

To test whether that relationship between planting date and ground cover holds, I graphed each grass species separately.

The relationship looks strongest for cereal rye, which, if you'll notice, is planted over the shortest window of just six weeks. It also seems to hold true for oats, but we probably don't have enough data for annual ryegrass.

Effect of Prior Crop

The crop previous to the cover crop is obviously what determines planting date to a large extent. Below you can see confirmation of our expectations that early harvested crops like small grains, canning crops, and dry beans tend to allow for more cover crop growth.

Next Steps

I expect to see at least 100 more photos in spring 2020. At that point, maybe we'll see the relationship between planting date and coverage strengthen for the overwintering species like cereal rye. Since most of the winter-killed species are planted early in the season, the exact timing might be less critical for the species represented in this preliminary data set. With more data, we may also be able to establish a threshold for this application to evaluate qualitative success of cover crops, for example, <10% ground cover = "Unsuccessful", 10-30% ground cover = "Moderate success", 30-55% ground cover = "Good success", and >55% = "Excellent establishment".